Electronic equipment employing digital techniques, such as computing devices, and other electronic or electrical equipment such as switching power supplies cause electromagnetic interference (EMI) which can both be radiated and conducted. EMI conducted over the power leads for the equipment occurs in a common mode and in a differential mode. Most of the conducted EMI is present as the common mode voltage which is measured with respect to both the line and the neutral conductors relative to ground. The differential mode conducted EMI is measured as a voltage between the line and the neutral conductors.
Designers and users of such equipment must be concerned with suppression of conducted EMI or noise because the noise can interfere with proper operation of other equipment which could be necessary to health and safety. Furthermore the U.S. Federal Communications Commission (FCC) and others have established regulations and standards regarding EMI emissions. For example, FCC Rules, Part 15, Subpart J concerns reduction of the interference potential of electronic computing devices. It is also desirable to protect equipment from outside noise, and to attenuate conducted noise without regard to its source.
It is known to place a power line interference filter between the power source and the equipment causing the EMI. Such a filter includes a common mode inductor having a core of permeable material with two windings, one winding part of the line conductor and the other winding part of the neutral conductor, and a differential mode inductor wound in one or both of the line and neutral. Such a filter also typically includes a first differential mode capacitor connected across the line and neutral between the common mode inductor and the power source, a second differential mode capacitor connected across the line and neutral between the inductors and the equipment, and a common mode capacitor connected from each of the line and neutral to ground. This filter arrangement requires 6 or 7 discrete components and a considerable amount of wire length which could either radiate EMI or receive radiated EMI. This is, in essence, a low pass filter. The impedance associated with the power source is relatively low, and at 60 Hz the impedance of the filter essentially matches that of the power source. However, at frequencies in the range of 10 kHz to 30 MHz and above generally associated with common mode noise, and frequencies of 10 kHz to 500 kHz generally associated with differential mode noise, the filter acts as an impedance mismatching network to prevent conduction at those high frequencies. Such a filter is also useful in D.C. applications, such as in a D.C. to D.C. converter, where the converter noise can propagate down the D.C. busses so as to adversely affect other components of the system.
From U.S. Pat. No. 3,170,133 it is known to provide a transformer with inner and outer core sections. The outer core section defines a window and has an air gap, and the inner core section is nested within the outer. The first and second windings of the transformer are wound about the cross sections of both core sections, the concept being that for lower values of exciting current, the resulting magnetic flux would tend to concentrate in the inner core which would saturate at predetermined values of exciting current above which the additional magnetic flux would concentrate in the outer core section. Additional windings are shown only about one of the core sections for testing purposes to permit determination of the magnetic characteristics of the individual core sections.